Recently, new-fashioned metal halide lamps have been developed. The metal halide lamps employ polycrystalline alumina ceramics in place of conventionally-used quartz as materials for arc tubes. The alumina ceramic arc tube allows the arc tube to have a higher bulb wall loading in a design due to high heat resistance as compared with a quartz arc tube, thereby realizing lamp properties of high efficiency and better color rendering.
As a conventional metal halide lamp, “a starter-containing lamp”, i.e., a lamp with a built-in starter, has been developed mainly. It can be lit by a simple copper-iron type inductance ballast that is used in a high intensity mercury lamp or the like.
For example, the starter-containing lamp can be lit even when employing an already-existing ballast that is used in a high intensity mercury lamp. Therefore, a starter-containing metal halide lamp having a quartz arc tube can be used easily as a lamp system, and further, the total cost can be lowered. Therefore, such metal halide lamps have been used widely as lamps for general indoor and outdoor lighting. The lamps have a rated lamp life of 6000 hrs to 12000 hrs.
In contrast, lamps lit by a ballast with a pulse voltage-generating function dominate as a recent metal halide lamp having an alumina ceramic arc tube. At the present time, these lamps are used mainly for indoor lighting in commercial spaces like shops. The lamps have a rated lamp life of 6000 hrs or more.
FIGS. 4 and 5 show examples of typical arc tube configurations of conventional metal halide lamps having alumina ceramic tubes containing no starter. FIG. 6 shows the overall configuration of a lamp employing the arc tube shown in FIG. 4.
An arc tube 33 shown in FIG. 4 has an arc tube container 34. The arc tube container 34 is composed of a discharge arc tube 35 formed of a polycrystalline alumina ceramic and a pair of narrow tubes 36 and 37 that are sintered at both end portions of the discharge arc tube 35. In the discharge arc tube 35, a pair of tungsten coil electrodes 38 and 39 extend from both inner ends thereof. The tungsten coil electrodes 38 and 39 are composed of two components of the tungsten electrode rods 40 and 41 and tungsten coils 42 and 43, respectively.
In the narrow tubes 36 and 37, power feeders 44 and 45 formed of niobium or a conductive cermet are sealed airtight by frit 46. Further, each of the tungsten electrode rods 40 and 41 is welded to be held at one end of each of the power feeders 44 and 45. When the power feeders 44 and 45 are formed of a conductive cermet, an external lead wire formed of niobium or the like may be welded and held at the other end of each of the power feeders 44 and 45.
In this configuration, the narrow tubes 36 and 37 are not filled entirely with the frit 46, but the frit 46 is stopped in the vicinity of the welded portions at the ends of the tungsten electrode rods 40 and 41. This configuration is necessary and indispensable for avoiding erosion of the frit 46 caused by a luminescent material during lighting of the lamp and for preventing cracks and breakages from occurring in the case of sealing directly the tungsten electrode rods 40 and 41 with the frit. Further, it can be said that this configuration realizes the metal halide lamp having an alumina ceramic tube.
This configuration consequentially forms spaces 47 and 48 between the narrow tubes 36 and 37 and the tungsten electrode rods 40 and 41. In the spaces 47 and 48, molybdenum coils 49 and 50 are wound around the tungsten electrode rods 40 and 41, respectively.
FIG. 5 shows another example of the arc tube, in which the narrow tubes 36 and 37 are configured differently from those shown in FIG. 4. In the arc tube shown in FIG. 5, the power feeders 44 and 45 are formed of a conductive cermet that is not eroded by a metal halide as a luminescent material. Therefore, the power feeders 44 and 45 are extended toward the electrodes and the molybdenum coils 49 and 50 are not provided.
However, the frit 46 is filled at a rear portion of the narrow tubes 36 and 37 also in the arc tube configuration for the same reason as described with reference to the arc tube shown in FIG. 4. Accordingly, the spaces 47 and 48 are formed on the side near a discharge space between inner surfaces of the narrow tubes 36 and 37 and the power feeders 44 and 45, while theses spaces are narrower as compared with those shown in FIG. 1.
The arc tubes shown in FIGS. 4 and 5 are filled with a metal halide such as DyI3, TmI3, HoI3, TlI and NaI as a luminescent material 51, mercury as a buffer gas, and a starting-assistance rare gas 52. As the starting-assistance rare gas 52 for the lamps containing no starter, argon Ar of 7 kPa or more but less than 13 kPa usually is used.
FIG. 6 shows an example of the overall configuration of a lamp having the arc tube 33 shown in FIG. 4. In a lamp 53 shown in FIG. 6, the arc tube 33 is provided in an outer bulb 54 formed of a quartz or a hard glass, and a lamp base 55 is fitted to the outer bulb 54. The outer bulb 54 is filled with a nitrogen-based gaseous mixture at a pressure of about 70 kPa. A shield quartz tube 56 for avoiding breakage in the outer bulb is provided around the arc tube 33.
FIG. 7 shows an example of a lighting circuit in which the copper-iron type inductance ballast is combined with the conventional starter-containing metal halide lamp having a quartz arc tube. A quartz arc tube 57 shown in FIG. 7 is provided with a pair of tungsten coils 58 and 59 at both ends thereof, and is attached to a tungsten auxiliary electrode 60 at one end portion thereof. The metal halide as a luminescent material 61 and the starting-assistance rare gas 62 are filled in the arc tube.
As the starting-assistance rare gas 62, argon (Ar) or a neon-argon penning gas (Ne and 0.1% to 2.0% Ar) of 7 kPa or more but less than 13 kPa usually is used. It should be noted that 0.1% to 2.0% Ar refers to a molar ratio or a pressure ratio of Ar. This can be applied to 0.5% Ar in a neon-argon penning gas (Ne and 0.5% Ar) as described below.
Use of the neon-argon penning gas provides Penning effect that metastable atoms of the main gas (Ne) excite or ionize the mixed gas (Ar). Ionization will lower the discharge-starting voltage and facilitate starting of the lamp.
In a completed lamp 63, the arc tube 57 is provided in a glass outer bulb 64. In the case where the neon-argon penning gas is used as the starting-assistance rare gas 62, a mixture of neon is filled at a pressure of about 70 kPa in the glass outer bulb 64 so as to prevent the neon gas from diffusing from the quartz arc tube 57 to the inside of the glass outer bulb 64.
A starter 65 is composed of a series circuit including a current switching element 66 formed by a glow starter or a nonlinear ceramic condenser, a current control resistor 67 and a bimetal switch 68. The series circuit is connected in parallel to the quartz arc tube 57.
During starting of the lamp, a high voltage pulse ranging from 1.5 kV to 2.0 kV is induced at a copper-iron type ballast 69 due to interruption of current caused by operations of the current switching element 66, thereby starting the quartz arc tube 57. The high voltage pulse for starting the arc tube is set to 2.0 kV or less so as to ensure a withstand voltage performance of the ballast that is used in a high intensity mercury lamp.
In order to widen the scope of the application of the metal halide lamp having an alumina ceramic tube and further advancing energy saving in the field of lighting equipment, the present inventors have pursued the research of developing more efficient metal halide lamps having alumina ceramic tubes. More specifically, the metal halide lamps can be used for conventional metal halide lamps having quartz arc tubes, which have been used widely for general indoor and outdoor lighting.
An object of the development is to develop a starter-containing metal halide lamp having an alumina ceramic tube. Such a metal halide lamp can be lit by a simple copper-iron type ballast used in a conventional high intensity mercury lamp or the like, having a rated lamp life of 6000 hrs or more.
In order to achieve the above-mentioned object, the present inventors aimed at employing a neon gas or a neon-based gaseous mixture having lower starting-operation voltage as the starting-assistance rare gas to be filled in the arc tube.
However, the experimental results showed that there arose a problem in a configuration in which the starter-containing metal halide lamp having an alumina ceramic tube was filled with the neon gas or the neon-based gaseous mixture as the starting-assistance rare gas. Specifically, in the configuration, the lamp life will be shortened due to cracks in the narrow tubes, while the starting-operation voltage was lowered. The details will be described at a later point in the specification.